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SUMMARY1. In the isolated colonic mucosa ofBufo arenarum, under special circumstances, there is a variable fraction of the short-circuit current (0-38 %) that is unaccounted for by either the Na or the Cl and bicarbonate transmembrane net fluxes.2. The hypothesis that a special kind of bicarbonate transport may account for the non-Na component of the short-circuit current was investigated. According to this, bicarbonate ions formed within the membrane await transport towards the mucosal solution within a compartment that does not undergo isotopic exchange with the serosal bathing solution. This kind of transport may be detected by a lowering of mucosal specific activity of bicarbonate but would not be revealed by the classic method of comparing the difference between the unidirectional fluxes with the shortcircuit current.3. The specific activity of bicarbonate was determined in the inside solution (initially bicarbonate-free) of ten normal and four everted colonic sacs incubated in an external medium (reservoir) containing a constant specific activity of bicarbonate. Comparison between membrane-tointernal solution bicarbonate flux and non-Na component of the shortcircuit current was carried out in two different ways: (a) by measuring the remaining short-circuit current in Na-free medium and (b) by determining simultaneously the Na net flux.4. Whatever the value of the short-circuit current and its non-Na component, there is no reduction of the specific activity of the bicarbonate appearing in the inside solution of the everted colonic sacs.* Present address: Physiological Laboratory, Downing Street, Cambridge, England. Reprint requests should be sent to this address.

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